Chapter 7 Lesson 2 - Earthquakes

Chapter 7 Lesson 2 - Earthquakes

Chapter 7 Lesson 2 - Earthquakes

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Presentation Transcript

1. Chapter 7 Lesson 2 - Earthquakes

2. How Do Earthquakes Happen? • An earthquake is defined as a sudden tremblingin the ground caused by something happening in the crust. • Most happen at the edges of plates, or plate boundaries. Very few happen in the center of plates. • About 80% happen along the edges of the Pacific plate. • Caused by plates colliding, sliding, or pulling apart. The rocks bend and stretch.

3. When plates move… • Great forces are exerted on the rocks. • First, the rocks bend and stretch. • Once they reach their limit, they break. • Faults form at or below the surface of the crust.

4. Faults • When rocks in the crust reach their breaking limit and crack. San Andres Fault

5. Earthquakes in NJ Why do most earthquakes occur in the mid-northern section of the state?

6. Normal Fault • Happens at divergent boundaries • Where plates pull apart • Rocks above the fault surface move down. • Ex: Sierra Nevada in CA

7. Reverse Fault • Happens at convergent boundaries • Where plates push together • Rocks above the fault move upward • Ex: Himalayas in India

8. Strike-slip Fault • Happens at transform boundaries • Plates slide past each other without moving up or down. • Where rocks slide past each other in different directions. • Ex: San Andreas Fault in CA

9. How Do Earthquakes Make Waves? • An earthquake starts the moment rocksbegin to scrape past each other along a fault. • It may be a new fault that forms at that moment or an oldfault that has already been formed.

10. Focus • The point where the earthquake starts, where rocks begin to slide past each other • It is usually below the surface • The sudden motion causes vibrations to spread out from the focus • These vibrations travel through the crust in the form of waves

11. Seismic Waves • The vibrations travel through the crust. • They reach Earth’s surface at a point directly above the focus called the epicenter.

12. Epicenter • Definition: The point on Earth’s surface directly above the focus of an earthquake • People can first feel the ground shaking. • This shaking is what causes most earthquake damage because it is so close to the focus. • As waves travel away from the focus they get weaker.

13. Aftershocks • After the first shaking, it may be quiet, and then aftershocks occur. • Aftershocks continue the damage of an earthquake. • Additional shaking and damage after the first shaking and relative quiet.

14. Earthquake Waves • Vibrations that are given off by an earthquake and travel through the crust are called seismic waves. • A seismic wave starts with shaking caused by rocks scraping against each other. • This shaking results in several kinds of seismic waves. • The waves travel at different speeds. • Three kinds: • Primary Waves – P waves • Secondary Waves – S waves • Surface Waves

15. Primary Waves – P waves • http://www.youtube.com/watch?v=2rYjlVPU9U4&feature=player_embedded • Accordion-like motion where rock squeezes together and spreads apart repeatedly. • Produces seismic waves that move in the same directionthat the rock is shaking. • The fastest seismic wave • The first to reach any faraway location

16. Secondary Waves – S Waves • http://www.youtube.com/watch?v=en4HptC0mQ4&feature=player_embedded • Shaking like a ruler held off the edge of a desk and “twanged” • Produces seismic waves that move in a different direction from the vibration • Travel slower than primary waves • They are the second to arrive at any given faraway location.

17. Surface Waves • Causes Earth’s surface to heave up and down like an ocean wave, or sway from side to side. • Tears apart structures built on the surface as the land stretches and compresses.

18. When a 7.0-magnitude earthquake rocked the south island of New Zealand one train track was permanently altered in what looks more like a Photoshopped image or a cartoon gag than an actual photograph.

19. Seismograph • Definition: A sensitive device that detects the shaking of the Earth’s crust during an earthquake. • Used by scientists to study earthquake waves • Shows patterns in the waves. • Can identify the P waves, S waves, and surfacewaves from studying the readings of the seismograph.